This invention relates generally to radar antenna systems and particularly to a five-port monopulse feed structure.
Using electromagnetic energy for the detection and location of objects, radar systems employ the well known method of transmitting a signal in a direction of interest and analyzing any echos. Objects in the path of energy emanating from the radar antenna redirect energy back to the antenna where it is processed at four receive ports through a comparator to determine positional information.
Many radar applications employ a special type of radar system called an amplitude-comparison monopulse system. Able to achieve the high precision required in such applications as weapon control and missile-range instrumentation, the monopulse system transmits a single pulse of energy in a very narrow beam toward a target and then analyzes the pattern of reflected energy according to its placement about the beam axis. When the reflected energy returns in a pattern symmetrically disposed about the beam axis, the beam is pointed right at the target. When the pattern is off center, the beam is pointed correspondingly off target.
The manner in which the pattern of reflected energy is disposed about the beam axis is deduced using four separate receiving structures called waveguide horns. These four receive horns define four openings, or ports, placed symmetrically about the feed point of a parabolic reflector. This four-port receive array is interconnected to the rest of the monopulse system by suitable waveguide components that define a path for electromagnetic waves to follow from the antenna to other components in the system.
Reflected energy returning to the antenna excites each of the four horns according to how the pattern of reflected energy is disposed about the beam axis. This energy then propagates along the waveguide components to signal processing components that precisely calculate target position by comparing the relative amplitudes of the energy received by each of the horns.
To visualize this, imagine four water glasses grouped tightly together under a water faucet. Water from the faucet fills the glasses at the same rate when the glasses are grouped symmetrically about the central axis of the flow of water. This is analogous to a monopulse radar beam being right on target with the four horns grouped symmetrically about the central axis of the reflected energy.
But just as the water glasses fill at different rates if the flow of water is off center, so does the energy received by each of the four horns vary when the target, and therefore the reflected energy, is off center. By carefully analyzing the relative amplitudes of reflected energy received through each of the four horns, the amount the target is off center can be precisely determined.
The horns are carefully positioned to do this. They are positioned at just the right spacing relative to each other and relative to the focal point of the parabolic reflector, and this enables accurate analysis of a narrow beam of reflected energy to determine placement about the beam axis. In this manner, a conventional amplitude-comparison monopulse radar system uses a four-horn array in conjunction with a parabolic reflector to achieve precise weapon control and missile-range instrumentation. This invention improves upon the four-horn array.
In existing amplitude-comparison monopulse radar systems, electromagnetic energy (RF) of a frequency suitable for radar applications is transmitted through the same four horns that are used to receive the reflected energy. What is called a circulator, or functionally equivalent transmit/receive (TR) device, properly routes the signal to enable the four horns to serve this dual purpose. Thus, a single RF pulse is generated and routed via the TR device to the four horns where it is transmitted toward a target, reflected back to the horns, and then routed once again via the TR device to signal processing components.
Although this arrangement manages to employ the four horns in both the transmit and receive mode, it has certain drawbacks related to the TR device. These include the added cost, size, and volume of a circulator or other TR device, as well as the power limitations and the complex waveguide plumbing required.
And while some designs employ a separate feed for transmitting energy, they upset critical spacing of the four receive horns and result in a feed structure unsuited for precise monopulse work with a parabolic reflector.
Consequently, it is desirable to have a new feed structure for amplitude-comparison monopulse radar applications that alleviates these concerns--one that eliminates the need for a circulator or other TR device, retains essential features and placement of the four-horn array, and adapts to use with a conventional parabolic or other concave reflective surface.